1. Field of the Invention
Polymers having high-temperature characteristics are required to improve the performance and to reduce the weight of industrial materials in electronic devices, aeronautical equipment and some machinery. The polyamides and polyesters are polymers known to have the required mechanical strength, dimensional stability, low coefficient of thermal expansion, and electrical insulation properties in addition to high-temperature resistance.
The preparation of high performance polymers, however, requires cure temperatures in excess of 200xc2x0 C. This leads to high tooling costs, high processing costs, and processing induced thermal stresses that can compromise component durability. The process of this invention allows the curing of high performance polyimides and polyesters at or near room temperature. This invention enables the cure of high performance polyimides and polyesters at or near room temperature by using ultraviolet light or some other radiation sources, such as electron beams rather than heat to provide the cure energy. Specifically, this invention relates to the Diels-Alder cyclopolymerization of a photochemically generated diene with dienophiles, such as di(acrylates), tri(acrylates), di(methacrylates), tri(methacrylates), and mixtures thereof with monoacrylates. Irradiation of aromatic diketones produces two distinct hydroxy o-quinodimethane (photoenol) intermediates. The intermediates are trapped via a Diels-Alder cycloaddition with appropriate dienophiles, e.g. di(acrylates) and/or tri(acrylates) to give the corresponding polyesters in quantitative yields. When acrylates such as di(acrylate) or di(methacrylate) are used as the bisdienophile, the resulting polyesters of this invention were found to have glass transition temperatures, (Tg), as high as 200xc2x0 C.
2. Description of the Prior Art
The preparation of high performance polymers such as the polyimides and polyesters are typically prepared by condensation reactions. In the case of polyimides, the reaction involves diamines and dianhydrides or dianhydride derivatives e.g., the diester of tetracarboxylic acids. This process suffers from several problems in that aromatic diamines are toxic, mutagenic, or carcinogenic. Safe handling and disposal of these material requires the implementation of costly engineering controls. Further, processing of condensation reaction systems also can be a problem, since this chemistry leads to low molecular weight by-products, e.g., water and alcohols. Evolution of these by-products and high processing temperatures lead to voids and defects in the polymer and composites prepared with these polymers.
Some of these processing problems can be overcome, however, by combining addition chemistry with condensation chemistry, as is the case for PMR-15 polyimides. With this approach, low molecular weight oligomers (short chain polymers) are prepared by the condensation of diamines with dianhydrides or their derivatives and a suitable endcapping group. The endcaps undergo a cross-linking reaction at high temperatures (typically in excess of 300xc2x0 C.) to provide a polymer network having good solvent resistance and high temperature performance. Prior to cross-linking, however, the imide oligomers are somewhat fluid and volatile condensation by-products can be removed from the polymer. While this approach overcomes some of the processing difficulties, it requires higher processing temperatures and monomer toxicity is still a concern.
It is known in the prior art that Diels-Alder polymerization reactions have been used to prepare high performance polymers such as the polyimides and polyacrylates. Typical Diels-Alder reactions used to obtain polyimides have involved the reaction of a bismaleimide with a suitable bisdiene such as a bisfuran. Other Diels-Alder reactions use a bisdiene precursor, such as a bis(benzocyclobutane), that forms the bisdiene upon heating to temperatures of 250xc2x0 C. or higher. Using these Diels-Alder cyclopolymerization reactions overcome the health and safety problems associated with other methods of preparing polyimides, since these reactions do not involve the use of aromatic amines as one of the reactants. However, these methods still require high cure and processing temperatures, see, for example, U.S. Pat. Nos. 5,338,827; 5,322,924; 4,739,030 and the Annual Reviews in Materials Science, 1998, 28, 599-630 by M. A. Meador.
In the case of polyesters, these systems are generally prepared by a polycondensation process involving the reaction of diols and diacids or diesters producing water or alcohol as byproduct. Unsaturated polyesters are similarly prepared with the exception that the diacids are unsaturated. It is known also that ethylenically unsaturated compounds, and in particular acrylate derivatives, can be polymerized by irradiation with ultraviolet light in the presence of a photoinitiating system. The photoinitiating system includes a diaryl ketone photoinitiator and a coinitiator, i.e. a molecule that serves as a hydrogen donor. The coinitiators are typically alcohols, or ethers which have available hydrogens attached to carbon atoms adjacent to heteroatoms.
The unique feature of this invention is that the process employs energy from ultraviolet light, rather than heat to form the polymers. While other radiation curable polymers have been developed, these methods employ either free radical or cationic-based polymerization chemistries. The present invention utilizes photochemically generated dienes (not free radicals or carbocations) and standard Diels-Alder cycloaddition chemistry in the polymerization process.
More specifically, this invention relates to polyesters i.e. polyacrylates and to the method of preparing these polymers derived from the photochemical cyclopolymerization of stoichiometric amounts of at least one aromatic diketone having the formula: 
wherein Ar is the same or a different aromatic or substituted aromatic radical e.g., a lower alkyl substitutent and R is the same or a different radical selected from the group consisting of hydrogen, aromatic radicals, substituted aromatic radicals, lower alkyl radicals of 1 to 8 carbons, O2CR1 and xe2x80x94OR2 radicals where R1 and R2 are the same or different organic radicals selected from the group consisting of lower alkyl radicals of 1 to 8 carbons e.g. 1 to 4 carbons, aryl and substituted aryl radicals and x in the diketone formula is selected from the group consisting of nil, oxygen, sulfur, xe2x80x94Cxe2x95x90O, xe2x80x94CH2xe2x80x94, alkyl radicals of 1 to 8 carbons, ether radicals, aryl radicals and substituted aryl radicals with at least one dienophile selected from the group consisting of di(acrylates), tri(acrylates), di(methacrylates), tri(methacrylates) and mixtures of monoacrylates or mono(methacrylates) with the di(methacrylates), tri(methacrylates), di(acrylates) and/or tri(acrylates) wherein the monoacrylates range from 0 to about 25 molar percent of the mixture to obtain polyacrylates having glass transition temperatures (Tg) as high as 200xc2x0 C., high thermal-oxidative stability and decomposition-stability temperatures ranging up to about 300xc2x0 C.
Accordingly, it is an object of this invention to employ energy from ultraviolet light rather than heat to obtain polyesters having glass transition temperatures as high as 200xc2x0 C.
It is another object of this invention to provide a novel method of preparing polyesters at ambient temperatures by using radiant energy to photochemically cyclopolymerize aromatic diketones and one or more acrylic dienophiles.
It is another object of this invention to provide a method of preparing radiation curable polyesters that do not have the health risk associated with conventional methods.
It is a further object of this invention to provide polyesters and a novel process of preparing cured polyesters by using radiation energy at ambient temperatures to obtain acrylic polymers derived from the polymerization of at least one aromatic diketone and acrylic dienophiles without using free radical or cationic polymerization methods.
These and other objects of this invention will become apparent from a further and more detailed description of the invention as follows:
This invention enables the curing of high performance polymers at or near room temperature by using ultraviolet light (or some other radiation sources, such as electron beams) rather than heat to provide the cure energy. In general, the invention involves the Diels-Alder cyclopolymerization of photochemically generated bisdienes with acrylic dienophiles. The general chemistry is described in Scheme 1, for a representative polyimide and polyacrylate. The irradiation of the aromatic diketone produces two distinct hydroxy o-quinodimethane (photoenol) intermediates. These intermediates are trapped via a Diels-Alder cycloaddition with appropriate dienophiles, e.g., di(acrylate), added prior to irradiation, to give the corresponding polymers in quantitative yields. When di(acrylates) or tri(acrylates) are used as the dienophile, the resulting polyesters have glass transition temperatures, (Tg) as high as 200xc2x0 C. depending upon the structures of the diketone and acrylates. Recent lab work has demonstrated that good polyacrylate films can be prepared by ultra-violet radiation of high solids content varnishes of the appropriate monomers in a small amount of various solvents, e.g. cyclohexanone, dimethyl formamide, N-methylpyrollidone and the like.
The general chemistry for the preparation of either the polyesters or polyimides from Diels-Alder trapping of photochemically generated bisdiene intermediates is shown (Schemel) as follows: 
For purpose of this invention, the other diketones used in preparing the polyacrylates, as in Scheme 1, include the following seven aromatic diketones: 
wherein Ar is the same or a different aromatic or substituted aromatic radical e.g. lower alkyl substitutents and R is the same or a different radical selected from the group consisting of hydrogen, aromatic radicals, substituted aromatic radicals, lower alkyl radicals of 1 to 8 carbons, O2CR1 and xe2x80x94OR2 radicals wherein R1 and R2 are the same or different radicals selected from the group consisting of lower alkyl radicals of 1 to 8 carbons, e.g. 1 to 4 carbons, aryl and substituted aryl radicals, and X in the diketone formulae is selected from the group consisting of nil, oxygen, sulfur, xe2x80x94Cxe2x95x90O, CH2, primary, secondary or tertiary alkyl radicals of 1 to 8 carbons, aromatic radicals, substituted aromatic radicals, primary, secondary or tertiary ethers, poly(ethers), esters, poly(esters) and poly(aryls), having the formula: 
wherein n has the value of 1 or 2, and X in the poly(aryl) formulae is a lower alkyl substitutent or nil.
The bisacrylates and bis(methacrylates) have a formulae selected from the group consisting of: 
wherein X is selected from the group consisting of oxygen, Cxe2x95x90O, SO2, CH2, nil, ether radicals, poly(ether) radicals, ester radicals, polyester radicals, aromatic and poly(aromatic) radicals; and R1 is selected from the group consisting of hydrogen, alkyl radicals of 1 to 8 carbons; and R2 is selected from the group consisting of alkyl(primary, secondary, or tertiary) radicals, ether radicals, poly(ether) radicals, ester radicals, and poly(ester radicals).
In addition to di(acrylates) and tri(acrylates), any of the di(methacrylates) or tri(methacrylates) can be used as the dienophile either alone or as a mixture with the tri(acrylates) and di(acrylates) in any molar ratio and further with the monoacrylates or mono(methacrylates) wherein the end-cap monoacrylates range from 0 to about 25 molar percent of the mixture. Structures of these tri(acrylic) dienophiles include, for example, the following: 
wherein R in the tri(acrylic) dienophiles is selected from the group consisting of hydrogen and CH3 and X is selected from the group consisting of nil, oxygen, xe2x80x94CH2, and xe2x80x94Cxe2x95x90O radicals.
Examples of the end-cap monoacrylates and methacrylates, known in the art include monomers, such as acrylic and methacrylic acids, and the amides, esters, and salts thereof. Specific mono(acrylic) monomers include, for example, methyl acrylate, ethyl acrylate, methyl methacrylate, butylacrylate, hydroxy ethylacrylate, hydroxy propylacrylate, the glycol acrylates, e.g. hexamethylene glycol dimethacrylate, allyl methacrylate, diallyl methacrylate, and the epoxy acrylates, e.g. glycidyl methacrylate and the like.
More specifically, the polyesters of this invention are derived from the photochemical cyclopolymerization at ambient temperatures of approximately stoichiometric amounts of at least one
(a) aromatic diketone selected from the group consisting of 
wherein Ar is the same or a different aromatic or substituted aromatic radical e.g. lower alkyl substitutents and R is the same or a different radical selected from the group consisting of hydrogen, aromatic radicals, e.g. substituted aromatic radicals, lower alkyl radicals of 1 to 8 carbons, O2CR1 and xe2x80x94OR2 radicals wherein R1 and R2 are the same or different radicals selected from the group consisting of lower alkyl radicals of 1 to 8 carbons e.g. 1 to 4 carbons and aryl and substituted aryl radicals, and X in the diketone formulae is selected from the group consisting of nil, oxygen, sulfur, xe2x80x94Cxe2x95x90O, xe2x80x94CH2, alkyl radicals of 1 to 8 carbons, ether or poly(ether) radicals, ester or poly(ester) radicals, and aryl or poly(aryl) radicals with at least one
(b) dienophile selected from the group consisting of di(acrylates), tri(acrylates), di- and tri(methacrylates) and mixtures of di(acrylates) and/or tri(acrylates) and/or di- and tri(methacrylates) with monoacrylates in molecular ratios of 0 to about 25 molar percent of the mixture to obtain polyacrylates having glass transition temperatures (Tg) ranging up to about 200xc2x0 C., high thermal-oxidative stability and decomposition-stability temperatures ranging up to about 300xc2x0 C.
Preferably, the polyesters of this invention are derived by a process of photochemically cyclopolymerizing with ultra-violet light at ambient temperatures approximately stoichiometric amounts of an
(a) aromatic diketone having the formula: 
wherein Ar is the same or a different aromatic or substituted aromatic radical and R is the same or different radical selected from the group consisting of hydrogen, aromatic radicals, lower alkyl radicals of 1 to 8 carbons, O2CR1 and xe2x80x94OR2 radicals wherein R1 and R2 are the same or different radicals selected from the group consisting of lower alkyl radicals of 1 to 8 carbons, and aryl radicals, and
(b) at least one acrylic dienophile selected from the group consisting of di(acrylates), tri(acrylates), di(methacrylates), tri(methacrylates) and mixtures thereof in any molar ratio with 0 to about 25 molar percent of at least one end-cap monoacrylate or mono(methacrylate).
The following examples illustrate the novel process of obtaining polyesters and polyimides by photochemically cyclopolymerizing diketones and dienophils at ambient temperatures.